This invention relates to sewing machines and, more particularly, to a system for use in a high speed industrial sewing machine to provide balanced stitching in the back-tack mode at high speed.
When sewing a seam on a work piece, it is a common practice to provide locking stitches at the leading and trailing edges of the seam. For purposes of the following description, "back-tack" refers generally to the locking stitches at both the leading and trailing edges of the seam, "first tack" refers specifically to the locking stitches at the leading edge of the seam, and "last tack" refers specifically to the locking stitches at the trailing edge of the seam. The first tack is formed by providing a fixed number of stitches in the forward direction followed by the same number of stitches in the reverse direction. Thereafter, the seam is sewn to its end. The last tack is formed by sewing a fixed number of stitches in the reverse direction followed by the same number of stitches in the forward direction. Additionally, at the end of the last tack, a further number of stitches in the reverse direction may be sewn for a reverse trim operation. In the prior art, the back-tack operation in automatic sewing machines was typically performed by counting the number of stitches and providing feed reversal command signals after the appropriate number of stitches. While this technique may be satisfactory at low speeds of operation, as the sewing speed increases the system inertias and response delay times prevent the machine from instantaneously reversing, thereby creating an imbalance in the different directions of back-tack stitches.
It is therefore an object of the present invention to provide an automatic back-tack system for a high speed sewing machine.
It is a further object of this invention to provide such an automatic back-tack system wherein balanced back-tack stitching is attained.